The present invention relates to a method for bridging brief power outages in a matrix converter with several power line-side commutation capacitors and a power line-side switching unit, as well as to a device for carrying out the method.
After a brief power outage, the desired rotation speed of individual drives should be re-established as quickly as a possible. Without additional measures, long restart times occur because signal processing has to be reestablished and newly initialized and the motor has to be newly excited. When several drives have to perform mechanically coordinated movements, the controllability of the drives can be lost during a power outage, which can damage the mechanical parts, unless additional measures are implemented.
The aforedescribed problem can be eliminated, for example, with an uninterruptible power supply, also referred to as UPS. Individual drives or several mechanically coupled drives can be connected to these UPS to provide a reliable power source independent of the state of the power grid. However, this solution is complex and costly for individual drives and/or drives with several motors.
Converter technology employing a voltage intermediate circuit can protect converter-supplied drives in the event of brief power outages. This option is referred to as kinetic buffering.
During a power outage, a single drive is driven into the generator operating mode when braking, with the kinetic energy of the motor and the coupled driven machine making up for the losses in the motor and of the converter. This is accomplished by using a controller that controls the intermediate circuit voltage to a fixed value, for example, to 80% of its nominal value. The control variable is the torque setpoint, or with a field-oriented controller an addition to the torque setpoint, or with the drives having U/f curve control an addition to the setpoint of the frequency. The supply voltage for the signal processing circuit is generated either separately from a reliable source or from the DC intermediate circuit. As a result, the signal processing circuit and the controller remain active, so that the motor stays excited and can be accelerated again to the speed setpoint immediately after the line voltage has been reestablished.
With several coupled drives, all intermediate circuits are coupled with each other to allow an energy exchange therebetween. In the event of a power outage, the rotation speed of all drives is lowered so as to make up for the energy requirements. The necessary rotation speed relationships or phase relationships between the various drives remain unchanged until the line voltage is reestablished or the system comes to a stop. The supply voltage of the signal processing circuit is generated in the same manner as described above for single drives. The intermediate circuit voltage is regulated to a value below the nominal value. The control variable is the main velocity setpoint of the system.
The option “kinetic buffering” can only be used with converters having a DC voltage intermediate circuit or a DC current intermediate circuit. This option cannot be employed with matrix converter which lacks an intermediate circuit.
When using a single drive, a matrix converter controlling a motor is connected to a power supply via an optional power line filter or an electromagnetic compatibility filter. This power line filter is implemented, for example, by employing line chokes and commutation capacitors connected to the input terminals of the matrix converter. The commutation capacitors which can be connected in a Delta or Star configuration, are essential for the operation of the matrix converter. However, the chokes in the supply lines may optionally be omitted.
In a multi-axis drive, several of the aforedescribed individual drives are operated from the same power grid. In other words, the input side of each matrix converter includes commutation capacitors and chokes. The number of chokes can be reduced by connecting the matrix converters electrically in parallel at the commutation capacitors. With this circuit variant, only three chokes are required with a three-phase power line.
It would therefore be desirable and advantageous to provide an improved method and a device for bridging brief power outages, which obviates prior art shortcomings and is able to specifically with conventional matrix converters.